In a wireless communication system, a divide-by-three injection locked frequency divider (ILFD) is employed to perform frequency division on a differential input voltage signal (e.g., at 78 GHz) to generate a differential output voltage signal having a frequency (e.g., 26 GHz) that is one-third the frequency of the differential input voltage signal.
Referring to FIG. 1, a conventional divide-by-three ILFD includes a mixer circuit 11, a filter circuit 12 that is a voltage-controlled oscillator having an oscillator frequency, and first and second buffer circuits 13, 14. The mixer circuit 11 includes two transistors 111, 112, receives a differential input voltage signal, and then mixes the differential input voltage signal and a voltage at the drains of the transistors 111, 112 which has a frequency about twice the oscillator frequency, so as to output a differential mixed signal. The filter circuit 12 filters the differential mixed signal to output a filtered differential voltage signal that has a frequency being one-third of the frequency of the differential input voltage signal. The first and second buffer circuits 13, 14 cooperatively buffer the filtered differential voltage signal so as to generate a differential output voltage signal (vo1, vo2).
A frequency locking range of the conventional divide-by-three ILFD, which is a frequency range for the conventional divide-by-three ILFD to correctly perform the frequency division by three, is somewhat narrow, ranging from 77 GHz to 79 GHz in this example. When the injection locking range of the conventional divide-by-three ILFD deviates to a range of, for example, from 74 GHz to 76 GHz because of variation in manufacturing process or temperature, the conventional divide-by-three ILFD may fail to perform correct frequency division by three on a differential input voltage signal having a frequency of, for example, 78 GHz, which now falls outside of the frequency locking range.